Flow control



Nov. 5, 1968 G. P. JENNINGS FLOW CONTROL Filed Nov. 5, 1966 INVENTOR. G.P. JENNINGS A TTORNEYS T vN mm n t u u N at 7 I mzfiz Nm wmwuw zsootimM265 0% 2 ZINE i? w. m .U\|.\ nw :1. mm NH UN N United States PatentABSTRACT OF THE DISCLOSURE A method for controlling the suction pressureof a pump transporting liquid wherein the vapor pressure of thetransported liquid is determined and the suction pressure of the pump iscontrolled in response to the determined vapor pressure so as tomaintain the suction pressure at a value above the determined vaporpressure which will prevent vaporization in the pump. Apparatus forcontrolling the suction pressure of the pump includes means to determinethe vapor pressure of a liquid being transported through the pump andmeans to control the suction pressure of the pump in response to thevapor pressure determination.

This invention relates to flow control. In one aspect the inventionrelates to method and apparatus for controlling flow through a pump. Inanother aspect the invention relates to controlling the suction pressureof a pump.

The transportation of liquid and liquefied products in the petroleumindustry by long distance pipelines is widespread. Because of thepressure drop along the pipeline due to friction between the liquid andthe pipe, it is necessary to utilize pump or booster stations positionedalong the line at intervals of approximately 25 (to 75 miles in order toboost the pressure from station to station along the line. In theoperation of such stations, it is desirable to prevent vaporization ofthe material being pumped. Previously, particularly at unattended pumpstations, it has been necessary to maintain a pressure in the pumpsuction high enough to prevent vaporization of the most volatileproducts. This necessitates pumping the less volatile products at anunnecessarily high suction pressure, thus reducing the efficiency of theoperation. I have found that by changing the pipeline pump suctionpressure when the vapor pressure of the product changes, it is possibleto prevent vaporization of the more volatile products, and to permit theless volatile products to be pumped at lower pressures.

An object of the invention is to improve the efliciency of operation ofa products pipeline.

Another object of the invention is to prevent vaporization of the morevolatile products and permit pumping all products at the lowestpractical pump suction pressure.

A further object of the invention is to prevent vaporization in a pump.

Other aspects, objects, and the advantages of the invention are apparentin the written description, the drawing, and the claims.

According to the invention, there is provided method to control thesuction pressure of a pump transporting liquid, in response to the vaporpressure of the liquid being pumped to prevent vaporization in the pump.In a preferred embodiment of the invention, the vapor pressure of theliquid being pumped is determined by withdrawing a small liquid streamfrom the discharge of the pump, passing it through a vapor pressuremeasuring zone where the vapor pressure is measured, and then returningit to the suction of the pump. A vapor pressure signal responsive to thevapor pressure measured in the zone is produced and the suction pressureof the pump is controlled responsive to this vapor pressure signal.

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Further, according to the invention, there is provided apparatus forcontrolling the suction pressure of a pump transporting liquid,comprising means to determine the vapor pressure of the liquid beingtransported through the pump and means to control the suction pressureof the pump in response to the determined vapor pressure at a valueabove the determined vapor pressure sufficient to prevent vaporizationin the pump.

FIGURE 1 is a schematic diagram of a suction pressure control system fora pump powered by a constant speed motor.

FIGURE 2 is a schematic diagram of a suction pressure control system fora pump powered by a variable speed engine.

FIGURE 3 is a typical flow nozzle in cross-section.

In FIGURE 1, pump 11 is provided with suction 12 and discharge 13. Asmall stream of liquid is piped from discharge 13 through flowcontroller 14 into flow nozzle 15. After passing through flow nozzle 15,the stream is then piped to suction 12. Flow nozzle 15 is constructed insuch a manner that the liquid stream passing into it is restricted insuch a manner that the velocity of the stream is sufficiently high so asto reduce the pressure in flow nozzle 15 to the equilibrium vaporpressure of the liquid stream at the flowing temperature thereof. Inoperation, the object is to attain a throat pressure which is equal toor slightly below the flowing vapor pressure of the pipeline stream.Theoretically, it is only necessary to pass enough product through flownozzle 15 to just reach the flowing vapor pressure. However, in order tobe sure vaporization is occurring, 8. little extra flow is passedthrough flow nozzle 15. The vaporization which results produces arefrigeration efiect which super cools the small stream and thus lowersthe flowing vapor pressure of the small stream. In operation, advantagecan be taken of this refrigeration in order to determine the requiredflow rate. In one installation, flow controller 14 was adjusted untilthere was about a 6- F. temperature drop from ther mometer 30 tothermometer 31. Such a temperature drop indicates that vaporization isoccurring within flow nozzle 15. Since vaporization is occurring in thepressure of both liquid and vapor phases, the pressure in the nozzle istherefore the vapor pressure of the liquid.

Because of the vaporization occurring within flow nozzle 15, bubbles areformed and these bubbles do not immediately close and disappear when thepressure increases as the stream passing through it exits. Therefore,the piping between flow nozzle 15 and suction 12 should be be largeenough to keep the friction loss low and insure proper return of theliquid stream to the pump suction, thereby allowing proper vaporizationin flow nozzle 15.

Vapor pressure transmitter 16 communicates by means of pipe 25 with flownozzle 15 and measures the pressure of the liquid passing through flownozzle 15, produces a corresponding vapor pressure signal, and transmitsthis vapor pressure signal to suction pressure controller 17. The -vaporpressure signal transmitted by vapor pressure transmitter 16 comprisesthe measured pressure in flow nozzle 15 plus a pressure constantcomprising a correction for the temperature drop between thermometers 30and 31 and a pressure allowance for the pipeline pump NPSH (net positivesuction head) requirement. This vapor pressure signal is used as the setpoint, or desired pump suction pressure, in suction pressure controller17. Suction pressure transmitter 23 measures the pressure in suction 12,produces a corresponding suction pressure signal, and transmits thesuction pressure signal to suction pressure controller 17. Suctionpressure controller 17 compares the vapor pressure signal received fromvapor pressure transmitter 16 with the suction pressure signal receivedfrom suction pressure transmitter 23 and sends a suction pressurecontrol signal responsive to this comparion to low select relay 18 tocause the actual suction pressure to achieve its set point valve. As thesuction pressure increases with respect to the set point value, thevalue of the suction pressure control signal increases. Dischargepressure transmitter 19 measures the pressure of discharge 13, producesa corresponding discharge pressure signal, and transmits this dischargepressure signal to discharge pressure controller 21. A set point ismechanically introduced into discharge pressure controller 21 bymechanical means 32. The discharge pressure signal is compared with theset point or desired value of pressure which is the maximum allowablepressure at which this point in the pipeline can be operated. Dischargepressure controller 21 then sends a discharge pressure control signalresponsive to this comparison to low select relay 18 to cause the actualdischarge pressure to achieve its set point value. As the pressure ofdischarge 13 increases, the value of the discharge pressure controlsignal decreases.

In low select relay 18, the lower in value of the pressure signalsreceived from controllers 17 and 21 is selected and passed therethroughto diaphram control valve 22, an air to open valve, whereby thethroughput of pump 11 is regulated responsive to the pressure ofdischarge 13 or responsive to the vapor pressure-suction pressurerelationship, depending upon which is the limiting condition in theoperation of this pumping system. The suction pressure control signal isthe signal which is normally selected by low select relay 18 to controlthe pump throughput. However, the discharge pressure control becomescontrolling at any time it is lower than the suction pressure controlsignal. In the system illustrated, the signals produced and transmitted,by transmitters 16, 19, and 23 and by controllers 17 and 21, arepneumatic pressure signals.

Low select relay 18 can be substituted with a high select relay whereinthe higher of the two signals received is transmitted to open or closevalve 22; however, the signals produced by transmitters 16, 19, and 23and controllers 17 and 21 must be reversed in value and air to openvalve 22 must be substituted with an air to close valve so that thesystem operates in harmony.

Differential pressure controller 24, i.e., a differential pressureswitch, is in communication with pipe 25 and suction 12 to protectagainst an instrument failure in pressure transmitters 16 or 23 orsuction pressure controller 17, and operates to shut off the pump powermeans when the positive differential between the pressure in suction 12and the pressure in pipe 25 is too small. A suitable instrument for thisfunction can be a Differential Pressure Cell, Model 199, made by BartonInstrument Corp., Monterey Park, Calif.

Pressure transmitters 16, 19, and 23 can be Ashcroft, Model 1260, madeby Manning, Maxwell & Moore, Stratford, Conn. Pressure controllers 17and 21 can be those made by Taylor Instrument Companies, Rochester, NewYork, and described in their catalog as 402RF0151 and 402RF0051,respectively. Low select relay 18 can be a Taylor, Type SK11359, made byTaylor Instrument Companies, Rochester, NY.

In FIGURE 2, the same numbers and descriptions apply as employed inFIGURE 1. FIGURE 2 illustrates the pump suction pressure control systemwhen the means employed for driving the pump comprises a variable speedengine. In low select relay 18, the lower in value of the pressuresignals received from controllers 17 and 21 is selected and passedtherethrough to pump engine speed control means 32 whereby thethroughput of pump 11 is controlled.

In FIGURE 3, a cross section of flow nozzle 15 is shown. The smallliquid stream withdrawn from discharge 13, as shown in FIGURE 1, entersthe flow nozzle through inlet 26. The velocity of the stream issufficiently high so as to reduce the pressure in throat 27 to theequalibrium vapor pressure of the liquid stream at the flowingtemperature thereof. The stream then exits through outlet 28 and isreturned to suction 12, as shown in FIGURE 1.

Thermometer 31 is inserted in well 29 and the temperature in throat 27is thereby measured.

Although the apparatus described above includes pneumaticinstrumentation, electrical, hydraulic, or any combination of thesethree can be used. If desired, controller 14 can be operated responsiveto measured values of temperature to maintain a desired temperaturedifferential between thermometers 30 and 31.

Reasonable variation and modification are possible within the scope ofthe invention, which sets forth method and apparatus for flow controlincluding controlling flow through a pump and controlling the suctionpressure of a pump in response to the vapor pressure of the liquid beingpumped to prevent vaporization in the pump.

What is claimed is:

1. A method for controlling the suction pressure of a pump transportingliquid, comprising:

withdrawing a small liquid stream from the discharge of said pump intoand through a vapor pressure measuring zone, and returning said streamto the suction of said pump;

measuring said vapor pressure of said liquid stream in said zone;producing a vapor pressure signal responsive to said vapor pressuremeasured in said zone; and controlling the suction pressure of said pumpin response to said vapor pressure so as to maintain said suctionpressure at a value above said vapor pressure to prevent vaporization insaid pump.

2. The method of claim 1 wherein, in said vapor pressure measuring zone,said liquid stream is passed through a flow nozzle whereby, within saidflow nozzle, the velocity of said stream is sutficiently high to reducethe pressure existing in said flow nozzle to the equilibrium vaporpressure of said liquid stream at the flowing temperature thereof.

3. The method claim 1 including:

controlling the flow of said small liquid stream to a value whichproduces partial vaporization of said stream in said vapor pressuremeasuring zone. 4. The method of claim 1 including: measuring saidsuction pressure and producing a suction pressure signal representationthereof;

producing a suction pressure control signal responsive to said vaporpressure signal and said suction pressure signal;

measuring the pump discharge pressure and producing a discharge pressurecontrol signal representation thereof;

selecting the lower valve of said suction pressure control signal andsaid discharge pressure control signal; and

controlling the throughput of said pump responsive to said lower value.5. The method of claim 1 including: measuring said suction pressure andproducing a suction pressure signal representation thereof;

producing a suction pressure control signal responsive to said vaporpressure signal and said suction pressure signal;

measuring the pump discharge pressure and producing a discharge pressurecontrol signal representation thereof;

selecting the higher value of said suction pressure control signal andsaid discharge pressure control signal; and

controlling the throughput of said pump responsive to said higher value.

6. An apparatus for controlling the suction pressure of a pumptransporting liquid, comprising:

a p p;

a flow nozzle capable of restricting a liquid stream whereby, withinsaid flow nozzle, the velocity of said liquid stream is sufficientlyhigh to reduce the pres sure existing within said flow nozzle to theequilibrium vapor pressure of said liquid stream at the flowingtemperature thereof;

conduit means to withdraw a small liquid stream from the discharge ofsaid pump into and through said flow nozzle and return said stream fromsaid flow nozzle to the suction of said pump;

flow control means in said conduit means used to withdraw said streamfrom said discharge into said fiow nozzle;

means to measure the temperature of said small liquid stream withdrawnfrom said discharge of said pump;

means to measure the temperature of said stream flowing through saidflow nozzle; and including:

means to produce a vapor pressure signal responsive to said equilibriumvapor pressure; and

means to control said suction pressure of said pump responsive to saidvapor pressure signal.

7. The apparatus of claim 6 wherein said means to control said suctionpressure of said pump responsive to said vapor pressure signalcomprises:

a suction pressure transmitter in communication with said suction ofsaid pump;

a suction pressure controller in communication with said vapor pressuretransmitter and said suction pressure transmitter;

a differential pressure control means in communication with the input tosaid nozzle pressure transmitter and said suction of said pump;

a discharge pressure controller;

a discharge pressure transmitter in communication with said discharge ofsaid pump and said discharge pressure controller; and

a low select relay in communication with said suction pressurecontroller, said discharge pressure controller, and means to control thethroughput of said pump.

8. The apparatus of claim 7 wherein said means to control the throughputof .said pump comprises:

a nozzle pressure transmitter in communication with said flow nozzle;and

said means to control said suction pressure of said pump responsive tosaid vapor pressure signal comprises:

a suction pressure transmitter in communication with said suction ofsaid pump;

a suction pressure controller in communication with said vapor pressuretransmitter and said suction pressure transmitter;

a dilferential pressure control means in communication with the input tosaid nozzle pressure transmitter and said suction of said pump;

a discharge pressure controller;

a discharge pressure transmitter in communication with said discharge ofsaid pump and said discharge pressure controller; and

a high select relay in communication with said suction pressurecontroller, said discharge pressure controller, and means to control thethroughput of said pump.

References Cited UNITED STATES PATENTS 2,859,768 11/1958 Teague 103-152,938,536 5/1960 Ehrenberg 103-11 2,944,488 7/1960 Meyer 103-352,950,682 8/1960 Kimmel 10335 WILLIAM L. FREEH, Primary Examiner.

